Hi! What I’m not quite sure about: did the neighbor just ask casually, or has he already presented you with final plans? He will have to decide at some point whether he wants 50 or 100 cm higher. And he should present you with complete, final plans that clearly show what he has in mind. That’s exactly what I did with my future neighbors, and I only deviated 15 cm from the height specification.
Best regards,
Andreas